Currently, within numerous medical contexts, respiratory movements constitute a parameter that it is necessary to take into account.
For example, in radiotherapy, the planning target volume (PTV), which is not modified during treatment, is deliberately overestimated in such a way as to take into account movements of the target tumor that are linked to unknown movements due to breathing. If this makes it possible to ensure complete irradiation of the tumor, this increases at the same time the irradiation of healthy tissue, which is not desirable for the patient's health.
Within the context of percutaneous punctures in interventional radiology, to reach his target, the radiologist asks the patient to hold his breath in a respiratory position that is similar to that of the tomodensitometric medical image, MRI, or echographic image that is used for highlighting the target. Consequently, the duration of the operation is clearly longer than if the position of the tumor were perfectly known.
These two illustrative examples explain the necessity for developing precise methods for simulation of the movement of the organs induced by breathing.
The respiratory movement has been considered for a long time as a perfectly cyclical movement, and consequently, it has often been characterized by a single variable representing, for example, the volume of air breathed in by the patient, or else a cutaneous marker positioned on the thorax. This approach is well known by one skilled in the art in the field in question.
In practice, this is not the case, as has been stated in particular in the article “A Real-Time Predictive Simulation of Abdominal Viscera Positions During Quiet Free Breathing”—“Simulation prédictive en temps-réel des positions des viscères abdominaux durant la respiration libre non forcée”—A. Hostettler et al., Progress in Biophysics and Molecular Biology, 103 (2010), pp. 169-184, ELSEVIER. In this article, in particular multiple markers are provided per axial plane of interest to monitor the movements of the skin, using a large number of optical markers or projection of structured light, for the purpose of studying or simply knowing the position of a patient's skin, without seeking to make a skin movement model.
Actually, in this article, the authors implemented a real-time monitoring of the movement of the skin at the level of the mid-sagittal plane using 8 optical markers (see Section 2.2), illustrating that, since the respiratory movement is primarily due to the combined action of the diaphragm and intercostal muscles, within the framework of free breathing, the action of these two effector muscles has not taken place in phase, and the amplitude of their respective movement changes over time. Consequently, it is not possible to model the respiratory movement or that of a target in the viscera using a single variable. For the same position of a cutaneous marker, a localized target in the viscera can have multiple significantly different locations.
Some of these methods for simulation of the movement of organs induced by breathing take into account the non-cyclical nature of the respiratory movement: the real-time knowledge of the position of the patient's skin is then obligatory for predicting the movement of the organs.
Actually, the skin position can be used to guide a digital model that predicts the position of the structures of interest located in the abdominal cavity, the thoracic cavity, or else the abdominal wall, such as, for example, in the above-mentioned article, in which the complete anterior surface of the abdomen and the thorax is used to guide the model, either via the use of the structured light, or via a number of cutaneous markers.
In addition, by way of the document “Bulk Modulus and Volume Variation Measurement of the Liver and the Kidneys In Vivo Using Abdominal Kinetics During Free Breathing”—A. Hostettler et al., Computer Methods and Programs in Biomedicine, 100 (2010), 149-157, furthermore, a process for determining variations in position and shape of the liver and kidneys during free breathing of a human subject is known, using a deformable model of the shape of the diaphragm to monitor the patient's skin. This document essentially goes back over the teaching that was already disclosed in the previously-cited document.
Although so-called “motion capture” methods exist today that make it possible to provide a real-time modeling of the skin of a moving subject, they require, however, all of the important material resources, making their implementation incompatible with an installation within an operating room (for example, the product that is known under the name “alignRT” of the ©vision RT Company). In addition, these known methods in general provide only an estimation or measurement of a fractional zone of the patient's skin and not the entirety of a zone of interest (for example, over essentially 360° and over a significant length in the craniocaudal direction).
In concrete terms, this surface data is difficult to obtain: during breathing, the skin moves nearly throughout and in different directions; only a portion of the surface that is in contact with the table remains immobile. Moreover, in numerous applications, it is necessary to monitor the skin position with an error that should not exceed on the order of one millimeter.
More specifically, essentially three different methods are currently known that are able to provide an exploitable result, but have all of the major drawbacks, namely:
a) Projection of structured light, monitored by camera and reconstruction of the surface of the skin:
In this case, a projection of structured light should be carried out from multiple sources in such a way as to cover the entire surface of the patient's skin. This poses a coverage problem at the junction of multiple projections. There should also be numerous cameras that make it possible to monitor the movements of the skin so as to ensure the complete visualization of these projections. A robust image processing algorithm should be used to calculate in real time the skin position that is then described as a cloud of more or less ordered points. Finally, this cloud of points should most often be resampled in such a way as to make it usable.
Be that as it may, with the patient lying on a table, this method makes it possible to locate only the portion of the skin that is visible with the cameras and on which the structured light is projected.
b) Monitoring by markers glued on the patient's skin and calculation of skin position by interpolation methods:
This method represents a simplification of the preceding method by reducing the number of points monitored in a significant way. Although faster, it keeps the same limits: the cameras should be numerous enough to monitor all of the visible markers. As a result, only the portion of the skin that is visible to the cameras and on which the markers are placed can be monitored. Finally, the interpolation will be less precise than the preceding method.
c) Monitoring of electromagnetic markers glued on the patient's skin (such as, for example, the Aurora system (filed name), developed by the NDI Company) and calculation of the skin position by interpolation methods:
In this case, there is no need for a camera, but the interpolation problems are similar. Moreover, the precision of the measurement of the position provided by the electromagnetic markers is lower than the preceding methods (from 1 to 5 mm), with the measurement error being in addition potentially increased by the presence of metal objects in the measuring fields.
This last limitation added to the current limitation of the number of simultaneously usable markers as well as to the frequency limited to 50 Hz actually makes it difficult to implement this method in the clinical routine.
Taking the preceding into account, the primary object of the invention is to propose a solution for modeling movements of the skin of a human subject that provides a reliable and precise three-dimensional simulation of at least one zone of interest, which evolves in real time and only requires a minimum number of measurements and limited processing power.